Acid has been used in the past for treating subterranean formations. In acid fracturing treatments, for instance, acid is placed in the fracture, at a distance from the wellbore, where it reacts with the face of the fracture to etch differential flow paths in the fracture faces. Normally, the acid is placed in the desired location by forming an acidic fluid on the surface and pumping the acidic fluid from the surface and down the wellbore above fracture pressure. Problems can be encountered, however, when using these acidic fluids.
First, in the pumping operation the acid is in contact with iron-containing components of the wellbore such as casing, liner, coiled tubing, etc. Acids are corrosive to such materials, especially at high temperature. This means that corrosion inhibitors must be added to the fluid being injected in order not to limit the amount of acid, and/or the time of exposure, that can be used during injection of an acid. Furthermore, acid corrosion creates iron compounds such as iron chlorides. These iron compounds may precipitate, especially if sulfur or sulfides are present, and may interfere with the stability or effectiveness of other components of the fluid, thus requiring addition of iron control agents or iron sequestering agents to the fluid.
Second, if the intention is to use the acid to treat a part of the formation at a significant distance away from the wellbore, this may be very difficult to accomplish because the acid will tend to react with the first reactive material with which it comes into contact. This may result in the acid reacting with materials closer to the wellbore and before the acid is in the desired position. Moreover, subterranean formations where the acid is used are typically high temperature environments and the higher the temperature the more reactive the acid is.
There have been several ways in which operators have dealt with these problems in the past. One method is to segregate the acid from the material with which reaction is not desired. This is done, for example, by a) placing the acid in the internal phase of an emulsion (so-called “emulsified acid”) and then either causing or allowing the emulsion to invert at the time and place where reaction is desired or allowing slow transport of the acid across the phase boundaries, or b) encapsulating the acid, for example by the method described in U.S. Pat. No. 6,207,620, and then releasing the acid when and where it is needed. There are problems with these methods, however. Although conventional emulsified acids are popular and effective, they require additional additives and specialized equipment and expertise, and may be difficult to control. Furthermore, as they tend to be “oil external phase” emulsions, the friction pressure associated with pumping these fluids tends to be relatively high. A problem with the encapsulated acids is that the location and timing of release of the acid may be difficult to control. The release is brought about by either physical or chemical degradation of the coating. Physical damage to the encapsulating material, or incomplete or inadequate coating during manufacture, could cause premature release of the acid.
A second method is to delay formation of the acid. Templeton, et al., in “Higher pH Acid Stimulation Systems”, SPE paper 7892, 1979, described the hydrolysis of esters such as methyl formate and methyl acetate as in situ acid generators in the oilfield. They also described the reaction of ammonium monochloroacetic acid with water to generate glycolic acid and ammonium chloride in the oilfield. However, these “acid precursors” are liquids, and these reactions take place very rapidly as soon as the “acid precursors” contact water.
More recently, the delayed formation of acid by the use of solid “polymeric acid precursors,” such as polylactic acid, has been developed wherein the solid polymeric acid precursor is injected into the formation and is allowed to hydrolyze so that “monomeric acids” are released at controllable rates. The use of such solid polymeric acid precursors is described in U.S. Patent Publication No. 2004/0152601A1, which is herein incorporated by reference in its entirety.